Replace unwrap with expect in setting RELM_THREADS

src/main.rs

@@ -27,7 +27,9 @@ fn main() {
   let cores = thread::available_parallelism()
     .map(|cores| cores.get() - 2)
     .unwrap_or(1);
-  relm4::RELM_THREADS.set(max(cores, 1)).unwrap();
+  relm4::RELM_THREADS
+    .set(max(cores, 1))
+    .expect("RELM_THREADS should not already be set");
 
   app.run::<App>(());
 }